1. Technical Field
This disclosure relates generally to the field of multi-specific binding molecules and therapeutic applications thereof and more specifically to a fusion protein composed of a CD86 antagonist binding domain, and another binding domain that is specific for a heterologous target, such as an IL-10 agonist, an HLA-G agonist, an HGF agonist, an IL-35 agonist, a PD-1 agonist, a BTLA agonist, a LIGHT antagonist, a GITRL antagonist or a CD40 antagonist, as well as compositions and therapeutic uses thereof.
2. Description of the Related Art
The human immune system generally protects the body from damage by foreign substances and pathogens. One way in which the immune system protects the body is by producing specialized cells, referred to as T lymphocytes or T-cells. Intercellular interactions between T-cells and antigen-presenting cells (APCs) generate T-cell costimulatory signals that in turn lead to T-cell responses to antigens. Full T cell activation requires both binding of the T-cell receptor (TCR) to antigen-MHC complex present on antigen-presenting cells and binding of the receptor CD28 on the surface of the T-cell to the CD86 and/or CD80 ligands present on antigen-presenting cells, particularly dendritic cells.
CD80 (also known as B7-1) was originally described as a human B-cell associated activation antigen and was subsequently found to be a receptor for the related T-cell molecules CD28 and cytotoxic T lymphocyte-associated antigen-4 (CTLA4). In later studies, another counterreceptor for CTLA4 known as CD86 (also known as B7-0 or B7-2) was identified. CD86 shares about 25% sequence identity with CD80 in its extracellular region. While CD80 and CD86 are generally believed to be functionally equivalent in their ability to initiate and maintain proliferation of CD4(+) T cells (Vasilevko et al. (2002) DNA Cell Biol. 21:137-49), and clinical data with a soluble CTLA4 Ig fusion protein that blocks this activity for both molecules has shown clinical benefit (Genovese et al. (2005) NEJM 353:114-1123), there is some evidence that specific inhibition of CD86 might be of benefit. For example, engagement of CD86 or CD80 has different effects on B cells. Specifically, CD80 has been shown to provide a negative signal for the proliferation and IgG secretion of both normal B cells and B cell lymphomas, while CD86 enhances the activity of B cells (Suvas et al. (2002) J. Biol. Chem. 277:7766-7775). There is also some evidence that engagement of CD80 on T cells is immunosuppressive (Lang et al. (2002) J. Immunol. 168:3786-3792; Taylor et al. (2004) J. Immunol. 172:34-39; Paust et al. (2004) PNAS 101:10398-10403) and that it may mediate further immunosuppression through PD-L1 (CD274) signaling on activated APCs or T cells (Butte et al. (2007) Immunity 27:111-122; Keir (2008) Ann. Rev. Immunol. 26:677-704). Accordingly, inhibition of CD86 in the absence of CD80 inhibition may be beneficial in the treatment of autoimmune and inflammatory disease as well as B cell lymphomas.
CTLA4 is a type 1 transmembrane glycoprotein of the immunoglobulin superfamily that is mainly expressed in activated T-cells, with some expression also being found in the CD4+CD25+ regulatory T-cell (Treg) subset. CD86 and CD80 are believed to be the only endogenous ligands for CTLA4. CTLA4 has been shown to bind CD86 and CD80 with greater affinity and avidity compared with CD28 (Linsley et al. (1991) J. Exp. Med. 174:561-69; Linsley et al. (1994) Immunity 1:793-801), and plays a key role as a negative regulator of T-cell activation. Specifically, binding of CTLA4 to CD80/CD86 leads to downregulation of T-cell responses and to the preservation of T-cell homeostasis and peripheral tolerance. This is believed to be due to both antagonism of CD28-dependent costimulation and directive negative signaling through the CTLA4 cytoplasmic tail. For a review of CTLA4 structure and function, see Teft et al. (2006) Annu. Rev. Immunol. 24:65-97.
As mentioned above, a productive immune response requires both engagement of TCR and binding of CD28 to CD80 and/or CD86. TCR binding in the absence of CD28 binding leads to T cells either undergoing apoptosis or becoming anergic. In addition, CD28 signaling has been shown to increase cytokine production by T cells. Specifically, CD28 stimulation has been shown to increase production of IL-2, TNFα, lymphotoxin, IFNγ and GM-CSF 5- to 50-fold in activated T cells. Furthermore, induction of lymphokine and/or cytokine gene expression by CD28 has been shown to occur even in the presence of the immunosuppressant cyclosporine (Thompson et al. (1989) Proc. Natl. Acad. Sci. USA 86:1333-1337). CD28 has also been shown to promote T cell survival by inducing upregulation of the anti-apoptotic BCL-XL (Alegre et al. (2001) Nature Rev. Immunol. 1:220-228).
Soluble forms of CTLA4 have been constructed by fusing the variable-like extracellular domain of CTLA4 to immunoglobulin constant domains to provide CTLA4-Ig fusion proteins. Soluble CTLA-4-Ig has been shown to prevent CD28-dependent costimulation by binding to both CD86 and CD80 (Linsley et al. (1991) J. Exp. Med., 174:561-69), and to inhibit costimulation of T cells and have beneficial immunosuppression effects in humans (Bruce & Boyce (2007) Ann. Pharmacother. 41:1153-1162). The CTLA4-Ig fusion protein abatacept is currently employed for the treatment of rheumatoid arthritis in cases of inadequate response to anti-TNFα therapy. However, not all patients respond to CTLA4-Ig and continued response requires frequent drug administration, perhaps in part because blockage of interaction of CD28 with CD86/CD80 is a weak inducer of Tregs and insufficient for blocking activated effector T cell responses in a disease milieu.